Method of and means for operating hot cathode tubes



. 11, 19344 H. smoN ETAL METHOD OF AND MEANS FOR OPERATiNG HOT CATH'ODE TUBES Filed Nov. 29, 1930 Y I I00 200 645 OR am PRIJM 6 THERMO-EECTRIO Patented Dec. 11, 1934 I UNITED STATES. PATENT OFFICE METHOD OF AND MEANS FOR OPERATING HOT CATHODE TUBES Hellmut Simon,

Berlin-Charlottenburg, and

Wolfgang Harries, Jena/Thuringen, Germany,

. assignors to Telefunken Gesellschaft fiir Drahtlose Tele eraphie m. b. IL, Berlin, Gen, many, a corporation of Germany Application November 29, 1930, Serial No. 499,125 In Germany November 30, 1929 6 Claims.

discharges of this kind comprising the use of heated cathodes are stable also where small currents are employed, as is well known from the investigations made on the low-potential are.

What is attended withserious difliculties in discharge tubes of the said sort is the attempt to make cathodes having a comparatively long life, inasmuch as the impact of positive ions upon the cathode causes disintegration of the electronemissive coat. It has been shown that such disintegration starts only above a certain velocity of the ions. In order to keep operation below such ion velocity, several conditions must be fulfilled in the operation of the discharge vessel. In the first place, it is necessary that the cathode possess an adequately copious electron emission independently of the heating occasioned by ions impacting thereon. The saturation current corresponding to the temperature of the cathode must be larger than the current flowing at the time in the tube. What follows therefrom is that the discharge should be initiated only when the cathode has attained the electron-emission temperature.

Disintegration of the electron-emissive cathode coat by high-speed ions may occur also when the gas-pressure goes beyond certain limits both in upward. and downward direction. For this reason,

it is of essential importance to choose the right filling pressure for tubes with a gas atmosphere.

In metal-vapor discharge vessels of this kind, such as mercury-vapor tubes, the vapor tension is governed by the temperature of the tube or tank walls. It has been found that the vapor pressure at room temperature is too low for most metalvapor discharge tubes equipped with a heated cathode in order to protect the latter from disintegration by high-speed ions in the presence of sufllcient emission.

According to this invention, in the operation of hot-cathode tubes with a metal-vapor atmosphere, a relatively long while is allowed to elapse after switching on the heating filament current before the discharge is initiated, in order to give the cathode a chance to assume the emission (Cl. Z50--Z7) temperature, so that the vapor pressure within the tube may attain a higher value. The thermal energy abducted from the heated cathode by virtue of radiation and conduction and yielded to the bulb wall, as a general rule, causes such an important temperature rise in the bulb that starting of the discharge is feasible without endangering the cathode. The time which must elapse between turning on the heating current and the initiation of the discharge in order to allow a sufllcient rise of temperature amounts to from three to five minutes in standard bulbs, though in other I cases it may be less or more. .The switching-in oi the filament current and initiation of the discharge with a difierence in time as required according to this invention may be automatically insured, preferably by switching means known in .the prior art, for example, by time-lag or retarded-action relays, or else by photo-electric arrangements which are rendered operable by the filament temperature of the heated cathode.

The invention is illustrated in the accompanying drawing wherein Fig. 1 shows the dependence of the fall of potential upon the vapor pressure; Fig. 2. the switching-in of the plate potential by the agency of a relay subject to the control of a photo-electric cell; and Fig. 3, the arrangement for switching-in of the plate potential by means of a time-lag relay in the heating or filament circuit.

Referring to Fig. 1, the three graphs numbered 1, 2 and 3 illustrate the inter-dependence of the fall oi potential across the arc and the vapor and gas pressure inside the discharge vessel. The shape of this graph is a function of the construction of the electrodes and the size and the shape of the vessel; and it furthermore depends on the pressure at which the gas or the vapor has originally been filled in the vessel. These curves go to make it clear that the arc potential accelerating the ions, in the presence of a pressure below 20x10-* mm. Hg column is high so that it will give rise to the destruction of the cathode surface by ion impact.

It will be noted that the grwhs, between 20 and 70 10- mm. Hg pressure evidence a minimum and that they rise again as the pressure grows. The pressure of from 20 to '70 X10 mm. Hg column, for the vessels useilin the experiments and for the initial pressures chosen for the gas filling, correspond to temperatures approximately from 25 to 60 degrees C.

Hence. the plate potential should only be applied when the gas content of the vessel has reached the temperature where the pressure has a value corresponding to the minimum point of the curves, and this time will vary with the form of the discharge vessel and the size of the initial pressure of the gas-filling. Hence, in the case of a. discharge vessel answering, say, the curve 1, the plate potential should onlybe switched in when the heat radiation of the filament has raised the gas-filling to a temperature between 40 and 45 degrees 0., for it is only then that the pressure of the gas corresponding to-the minimum in graph 1 is attained, namely, approximately 40x 10" mm. Hg column.

In order that such retarded switching operation may be automatically accomplished according to the invention, Fig. 2 shows a photo-electric cell 4 excited by the luminous radiation of the heated cathode 5 inside the discharge vessel 6, the currents or potential thus released being fed to the relay 8 by way of an amplifier 7. The said relay 8 by the aid of the contacts 9 and 10 will then close the circuit between the generator 11 and the consumer 12. The heating time of the cathode representing the time-lag element of the circuit, the relay 8 will onlybe caused to respond after the filament has attained a certain brightness.

In lieu of the photo-cell 4 it would also be teasible to provide a thermo-electric couple or element 4' responsive to heat radiation of the oathode or filament asshown in Fig. 3.

In the case of discharge vessels having heated cathodes which attain their electron-emission temperatureat a'high rate of speed, the plate circuit could be connected a definite length of time after the switching-in of the heating or filament circuit in that a'time-lag relay is made to effect the closure of, the consumer or load circuit.

The present invention is not limited to rectifler tubes, but may be used also in the operation of the most varying kinds of discharge vessels in which a discharge takes place having arc characteristics inside a metal-vapor atmosphere, between a heated cathode and one or more electrodes.

We claim:

1. The method of operating a mercury-vapor tube having a filamentary cathode and an anode, which comprises first heating the cathode until it has assumed the electron-emission temperature, then causing the radiations from the heated tric device for initiating the electron discharge between the heated cathode and the anode..

3. In combination a space charge device comprising a cathode and an anode, means for heating the said cathode to its electron-emission temperature, a space current source and an external circuit connected to said device, and means responsive to radiant energy emanating from said heated cathode for controlling the supply of space current to said device comprising a photo-electric device, and circuit closing means in said external circuit actuated by said photo-electric device.

4. In combination a space charge device comprising a cathode and an anode, means for heating the said cathode to its electron-emission temperature, a space current source and an external circuit connected to said device. and means re-' sponsive to radiant energy emanating directly from said heated cathode for controlling the supply of space current to said devicecomprising a thermo-electric couple, and circuit closing means in said external circuit actuated by said thermoelectric couple.

5. The method of operating a space discharge device having a filamentary cathode and an anode, which comprises heating the cathode to its normal operating temperature and controlling by means of radiant energy emanating directly from said heated cathode the initiation andcontinuous fiow of space current between anode and cathode.

6. In combination a space discharge device comprising a filamentary cathode and an" anode, a source for supplying heating current to said cathode, a space current source for said device,

and means responsive to radiant energy emanating directly from said heated cathode for contin-. uously controlling the supply of space current to said device.

HELLMUT SIMON. WOLFGANG HARRIES.

CJI 

